Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element, such as a stylus or cursor and transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. One model of this type of digitizer, called a "GRAPHBAR", employs a pair of "point" microphones, having generally circular receptivity patterns, mounted in spaced relation in an elongated generally rectangular housing. The housing or "bar" can be conveniently moved to a position adjacent an area in which the position of a movable element, containing a sound source, is to be digitized. The transit time of sound traveling from the source to each microphone is used, in conjunction with the speed of sound in air and known geometrical relationships, to compute the position of the movable element.
Through-the-air sonic digitizers have various advantages, one of which is the ability to obtain position information without the requirement for any particular type of working surface or space. No special medium is needed to carry signals. The "bar" can be positioned wherever the user desires, for example on a drawing placed on a table, on a piece of plywood, or on other media on which distance, area, or other geometrical measurements are to be made. The region that the microphones generally face is the "working region" of the digitizer.
Although through-the-air sonic digitizers have the indicated important advantages, they have limitations in some types of applications. One such limitation is a consequence of the speed of sound through air, which can limit the dynamic response, for example when the stylus is moved relatively quickly. Another such limitation is susceptibility to air current variations or ambient sonic noise in certain environments. A type of sonic digitizer that is less effected by these limitations is one that operates with the sonic energy, such as ultrasound, propagating through a solid medium such as a plastic or glass data tablet. The speed of ultrasound in such a medium is much higher than the speed of sound in air, so there can be a faster dynamic response. Also, air currents and ambient noise are not significant problems. However, with existing approaches to solid tablet ultrasonic digitizers, there is little flexibility of operation. A tablet surface of solid material, such as plastic or glass, through which the ultrasound can propagate, must be used as the work area, and transducers are connected to specific locations on the solid material. Reference can be made, for example, to U.S. Pat. Nos. 4,488,000 and 4,564,928 wherein an ultrasonic digitizer utilizes a plastic or glass tablet, and strips of polyvinyladine fluoride ("pvdf") are secured to the tablet for sensing ultrasonic energy that propagates in the tablet from the tip of a stylus.
It is among the objects of the present invention to provide a position determining apparatus which can operate with greater flexibility than existing solid tablet digitizers, and which can exhibit improved operation in various applications.